1. Field of the Invention
This invention relates in general to servo mechanisms and, more particularly, to servos for controlling the position of a playback head to a given playback track of a rotating information-bearing disc containing a plurality of such tracks. While the invention is not so limited, it is cast in the environment of apparatus for use with a magnetic disc having video information recorded thereon; and, in the interest of simplicity, the type servo employed is a static servo (i.e. one that drives to a given track position for the head, and thereafter holds that track position), rather than a dynamic servo which continuously seeks an optimal track position for the head in question.
2. Preliminary discussion as to the inventive static servo, and to various problems which it addresses:
As will appear below, the inventive static servo derives its control from each selected track of the disc in question. Such a technique is advantageous because it allows positive control to a track by means of that track. The derivation of such control, however, embraces certain problems, and such problems influence the operation of the servo. For example:
1. Given that the playback head is located at a first track, one would wonder how it would be possible for the servo to derive its control for positioning the head to a selected "different" track.
2. Given that the playback head is, however, en route to the selected "different" track, but is located within the essentially signal-free guard band that is usually found between tracks, one would also wonder how it would be possible for head-positioning to be brought under control of the selected "different" track.
3. Assuming the selected "different" track, to which the playback head is to be positioned, was recorded eccentrically, one would also wonder how it would be possible to position the head so that its output does not get amplitude modulated as the head picks up more, or less, signal from the eccentrically recorded track.
Absent the invention, the first of the above-listed items would manifest itself in having the playback head lock onto--and resist ever leaving--a given track. As to the second of the above-listed items, such would manifest itself by presenting ambiguous control to the positioning of the playback head. And, as to the third of the above-listed items, this would not only cause the indicated modulation of the playback head output, but (because head position control is derived from the track itself) could possibly result in aberrant operation of the servo.